1. Field of the Invention
The present invention relates to a transmission system for connecting modems which have various common transmission rates, and to means for adapting the transmission rate to the line quality. Such means provides, during a synchronization set-up phase of a communication, for:
exchanging between the modems relating to their transmission rates, PA1 providing a measurement of the quality of the transmission medium, PA1 changing the rate as a function of said measurement. PA1 a minimum exchange rate, rate.sub.-- min(exchange), which is the greater of the values rat.sub.-- min(local) and rate.sub.-- min(remote), where rate.sub.-- min(local) and rate.sub.-- min(remote) are the minimum acceptable rates for the local and remote modems; and respectively, PA1 a maximum exchange rate, rate.sub.-- max(exchange), which rate.sub.-- max(exchange) is the smaller of the values rate.sub.-- max(local) and rate.sub.-- max(remote), where rate.sub.-- max(local) and rate.sub.-- max(remote) are the maximum acceptable rates for the local and remote modems, respectively.
The invention also relates to a method of adapting the transmission rate to the line quality and relates to modems implementing said method.
2. Description of the Related Art
Transmission media are subjected to temporary disturbance (impulse noise, amplitude jumps) or permanent disturbance during the whole connection (white noise), which thus causes frequent resynchronizations or a high error rate to occur. The resynchronizations interrupt the data transmission for several seconds.
There is thus the problem of transmission rate adaptation between two modems (a local modem and a remote modem) as a function of the quality of the line and of the rate capacities of the remote modem. This is described in Japanese Patent Application no. 62-266581 of 23 Oct. 1987, published under number 1-109935. That Application implies that tile modems know each other perfectly well.
Currently, modems are proposed which are capable of transmitting data at 7 different rates: from 4800 bits/s to 19200 bits/s in steps of 2400 bits/s, for example. Thus the problem of adaptation to the right rate is much more acute.
The present invention proposes a system which permits connecting modems which have various rates, so that they can mutually adapt their rate.
Therefore, a system of the type described in the opening paragraph is characterized in that for connecting a local modem to a remote modem which includes a synchronization phase, it further comprises prior to the whole synchronization step defining:
The system is also used after the synchronization phase, for establishing the "rate.sub.-- trans" that being the transmission rate defined by the quality of the transmission medium.